- In a large bowl, combine the flour and salt. Add the butter and blend into the flour using a pastry cutter until the butter is pea-sized. Gradually add the water and mix just until the dough sticks together when pressed. Form into a disc. Cover with plastic wrap and refrigerate for 30 minutes.
- With the rack in the middle position, preheat the oven to 400°F (200°C).
- On a lightly floured surface, roll out the dough to 1/8 inch (3 mm) thick. Line a 9-inch (23 cm) tart pan with the dough and prick all over with a fork. Cover with foil and fill with pie weights.
- Bake for 30 minutes or until the edge of the crust is lightly golden. Remove the foil and pie weights. Turn the oven down to 350°F (180°C).
- Meanwhile, place the tomato slices on a wire rack set over a baking sheet to allow the excess liquid to drain. Sprinkle with the salt and let drain for 15 minutes. Pat dry with paper towels.
- Spread the mustard along the bottom of the tart. Layer the cheese, herbs and tomatoes on top.
- Bake for 20 minutes or until the tomatoes are hot and the cheese is slightly melted. Let cool for 15 minutes before serving.
Sweat the tomatoes with salt before cooking to release some water content. That will keep your crust from getting soggy.